Laser welding quality inspection method and laser welding quality inspection apparatus

ABSTRACT

A laser welding quality inspection method of a welded portion between a joining object and a joined object, when the joining object and the joined object are welded by being irradiated with a laser beam, the method includes: acquiring first data indicating a signal intensity of thermal radiation light radiated from the welded portion during the welding; acquiring second data indicating a signal intensity of plasma light radiated from the welded portion during the welding; and determining whether or not the welded portion includes an abnormality based on a comparison between the signal intensity of the thermal radiation light and the signal intensity of the plasma light which are acquired.

BACKGROUND 1. Technical Field

The disclosure relates to a laser welding quality inspection method anda laser welding quality inspection apparatus, and more specifically, todetermination of a welding abnormality during laser welding.

2. Description of the Related Art

As a laser welding quality inspection method of the related art, forexample, in Japanese Patent No. 3154177, a welding defect is determinedby utilizing a peak intensity of plasma light or reflection lightemitted from a welded portion during laser welding. Further, in JapanesePatent Unexamined Publication No. 2007-96442, a welding defect isdetermined by utilizing a time integrated intensity of each ofreflection light, plasma light, and infrared light from a joint portion,during laser welding.

SUMMARY

However, in a method of performing a determination of a welding defectduring laser welding by a peak intensity of welding light (thermalradiation light, plasma light and laser reflection light) generatedduring the laser welding of the related art, or an integrated value ofthe intensity of those types of welding light, when there is a clearwelding abnormality, it is possible to determine the welding defect, butwhen there is a minute welding abnormality, there is a problem that thewelding defect cannot be accurately determined. The inspection method ofthe related art still has room for improvement in terms of moreaccurately detecting an occurrence of the abnormality during the laserwelding.

Therefore, the disclosure is made to solve the above-described problemof the related art; and an object of the disclosure is to provide alaser welding quality inspection method and a laser welding qualityinspection apparatus that can determine a welding abnormality withhigher accuracy.

In order to achieve the above-mentioned object, according to anembodiment of the disclosure, a laser welding quality inspection methodis a welding quality inspection method of a welded portion between ajoining object and a joined object, when the joining object and thejoined object are welded by being irradiated with a laser beam, themethod including: acquiring first data indicating a signal intensity ofthermal radiation light radiated from the welded portion during thewelding; acquiring second data indicating a signal intensity of plasmalight radiated from the welded portion during the welding, anddetermining whether or not the welded portion includes an abnormalitybased on a comparison between the signal intensity of the thermalradiation light and the signal intensity of the plasma light which areacquired.

According to another embodiment of the disclosure, a laser weldingquality inspection apparatus is a welding quality inspection apparatusfor a welded portion between a joining object and a joined object, whenthe joining object and the joined object are welded by being irradiatedwith a laser beam, the apparatus including: a measurement device and awelding state determination device. The welding state determinationdevice includes a signal intensity acquisitor that acquires, from themeasurement device, first data indicating a signal intensity of thermalradiation light, and second data indicating a signal intensity of plasmalight which are radiated from the welded portion during the welding, anda signal intensity processor that executes processing of the first dataand the second data acquired by the signal intensity acquisitor. Thesignal intensity processor determines whether or not the welded portionincludes an abnormality based on a comparison between the signalintensity of the thermal radiation light and the signal intensity of theplasma light which are acquired.

As described above, according to the laser welding quality inspectionmethod and the laser welding quality inspection apparatus according tothe disclosure, by comparing the thermal radiation light and the plasmalight generated during the laser welding, the welding abnormality can bedetermined with higher accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall view illustrating a configuration of a laserwelding quality inspection apparatus according to an exemplaryembodiment of the disclosure;

FIG. 2A is a schematic view illustrating a welding state during normalwelding according to the exemplary embodiment of the disclosure, and isa sectional view of a welded portion at a time of a start of welding;

FIG. 2B is a schematic view illustrating a welding state during normalwelding according to the exemplary embodiment of the disclosure, and isa sectional view of a welding state at a time of completion of welding;

FIG. 3 is a graph illustrating setting of an output waveform of a laserbeam according to the exemplary embodiment of the disclosure;

FIG. 4A is a schematic view illustrating a welding state during abnormalwelding according to the exemplary embodiment of the disclosure, and isa sectional view of a welded portion at a time of a start of welding;

FIG. 4B is a schematic view illustrating a welding state during abnormalwelding according to the exemplary embodiment of the disclosure, and isa sectional view of a welding state at a time of completion of welding;

FIG. 5A is a view illustrating an external appearance of a melted andsolidified portion during normal welding according to an exemplaryembodiment of the disclosure;

FIG. 5B is graphs illustrating an irradiation output waveform of a laserbeam and a signal intensity of thermal radiation light during normalwelding according to the exemplary embodiment of the disclosure;

FIG. 6A is a view illustrating an external appearance of a melted andsolidified portion during abnormal welding according to an exemplaryembodiment of the disclosure;

FIG. 6B is graphs illustrating an irradiation output waveform of a laserbeam and a signal intensity of thermal radiation light during abnormalwelding according to the exemplary embodiment of the disclosure;

FIG. 7A is a diagram illustrating thermal radiation light during weldingof a welded product having a plurality of different welding states in awelding defect determination of the related art, and is a graphillustrating signal intensities of peaks in thermal radiation light of aplurality of welded products;

FIG. 7B is a diagram illustrating the thermal radiation light duringwelding of a welded product having a plurality of different weldingstates in the welding defect determination of the related art, and is anenlarged graph illustrating a portion where peaks of the signalintensities are equal to or less than 700 in the thermal radiation lightof a plurality of welded products;

FIG. 8 is a table illustrating a verification result of a welding statedetermination according to the related art;

FIG. 9A is a view illustrating an external appearance of a melted andsolidified portion of an abnormal welded product in the welding defectdetermination of the related art;

FIG. 9B is a graph illustrating a signal intensity of thermal radiationlight during welding of an abnormal welded product in the welding defectdetermination of the related art;

FIG. 10A is a view illustrating an external appearance of a melted andsolidified portion of a normal welded product in the welding defectdetermination of the related art;

FIG. 10B is a graph illustrating a signal intensity of thermal radiationlight during welding of a normal welded product in the welding defectdetermination of the related art;

FIG. 11 is a signal intensity of thermal radiation light and a signalintensity of plasma light during welding according to the exemplaryembodiment of the disclosure, and is a graph illustrating measurementresults of (a) a normal welded product, (b) abnormal welded product 1that is determined as a welding abnormality in the method of the relatedart, and (c) abnormal welded product 2 that is not determined as awelding abnormality in the method of the related art;

FIG. 12 is a block diagram illustrating a configuration of a weldingstate determination device in the laser welding quality inspectionapparatus of FIG. 1;

FIG. 13 is a diagram illustrating a flowchart of a determination processof the welding state of the laser welding quality inspection apparatusaccording to the exemplary embodiment of the disclosure;

FIG. 14 is an execution result of step S102 in the determinationflowchart of the welding state of FIG. 13, and is a view illustrating(a) an irradiation output waveform of a laser beam, (b) a signalintensity of thermal radiation light of a welded product having a minutewelding abnormality, and (c) a signal intensity of plasma light of thewelded product having a minute welding abnormality;

FIG. 15 is an execution result of step S103 in the determinationflowchart of the welding state of FIG. 13, and is a view illustrating(a) a signal intensity of the thermal radiation light during anextracted determination period and (b) a signal intensity of the plasmalight in the extracted determination period of a welded product having aminute welding abnormality;

FIG. 16 is an execution result of step S104 in the determinationflowchart of the welding state of FIG. 13, and is a view illustrating(a) a normalization signal of the thermal radiation light in thedetermination period and (b) a normalization signal of the plasma lightin the determination period of a welded product having a minute weldingabnormality;

FIG. 17A is an execution result of step S105 in the determinationflowchart of the welding state of FIG. 13, and is a graph illustrating acalculation result of a difference signal between a normalization signalof the thermal radiation light and a normalization signal of the plasmalight in a determination period of a welded product having a minutewelding abnormality;

FIG. 17B is an execution result of step S105 in the determinationflowchart of the welding state of FIG. 13, and is a graph illustrating adetermination of a welded product having a minute welding abnormality bya calculated difference signal;

FIG. 17C is an execution result of step S105 in the determinationflowchart of the welding state of FIG. 13, and is a graph illustrating adetermination of a normal welded product by a calculated differencesignal;

FIG. 18A is a diagram illustrating a difference signal between anormalization signal of the thermal radiation light and a normalizationsignal of the plasma light in a determination period of a welded producthaving a plurality of different welding states in the determination ofthe welding state according to the disclosure, and is a graphillustrating signal intensities of peaks in difference signals of aplurality of welded products;

FIG. 18b is a diagram illustrating a difference signal between anormalization signal of the thermal radiation light and a normalizationsignal of the plasma light in a determination period of a welded producthaving a plurality of different welding states in the determination ofthe welding state according to the disclosure, and is an enlarged graphillustrating a portion where the signal intensities of the peaks in thedifference signals of a plurality of welded products are equal to orless than 1.0; and

FIG. 19 is a table illustrating a verification result of a welding statedetermination according to the exemplary embodiment of the disclosure.

DETAILED DESCRIPTIONS

According to a first aspect of the disclosure, there is provided awelding quality inspection method that is a laser welding qualityinspection method of a welded portion between a joining object and ajoined object, when the joining object and the joined object are weldedby being irradiated with a laser beam, the method including: acquiringfirst data indicating a signal intensity of thermal radiation lightradiated from the welded portion during the welding; acquiring seconddata indicating a signal intensity of plasma light radiated from thewelded portion during the welding; and determining whether or not thewelded portion includes an abnormality based on a comparison between thesignal intensity of the thermal radiation light and the signal intensityof the plasma light which are acquired.

According to a second aspect of the first aspect of the disclosure,there is provided the laser welding quality inspection method, in whichthe determining whether or not the welded portion includes anabnormality based on a comparison between the signal intensity of thethermal radiation light and the signal intensity of the plasma lightwhich are acquired includes calculating a difference signal indicating adifference between the signal intensity of the thermal radiation lightand the signal intensity of the plasma light, and determining that thewelded portion includes an abnormality when the calculated differencesignal includes a peak having a signal intensity larger than a presetdetermination reference value.

According to a third aspect of the second aspect of the disclosure,there is provided the laser welding quality inspection method, in whichthe laser welding quality inspection method further includes: acquiringan irradiation output waveform indicating an intensity of irradiationlight of the laser beam by measuring the irradiation light of the laserbeam during the welding, in which the calculating a difference signalindicating a difference between the signal intensity of the thermalradiation light and the signal intensity of the plasma light furtherincludes, setting, as a determination period, a period during which theintensity of the irradiation light of the laser beam is constantlymaintained, based on the irradiation output waveform, and extracting thesignal intensity of the thermal radiation light within the determinationperiod and the signal intensity of the plasma light within thedetermination period respectively from the signal intensity of thethermal radiation light and the signal intensity of the plasma lightwhich are acquired, and in which the calculating a difference signalindicating a difference between the signal intensity of the thermalradiation light and the signal intensity of the plasma light includescalculating a difference between an intensity of the thermal radiationlight within the determination period and an intensity of the plasmalight within the determination period.

According to a fourth aspect of the third aspect of the disclosure,there is provided the laser welding quality inspection method, in whichthe calculating a difference signal indicating a difference between thesignal intensity of the thermal radiation light within the determinationperiod and the signal intensity of the plasma light within thedetermination period includes, calculating a normalization signal of thethermal radiation light and a normalization signal of the plasma lightby respectively normalizing the signal intensity of the thermalradiation light within the determination period and the signal intensityof the plasma light within the determination period, and calculating adifference signal indicating a difference between the normalizationsignal of the thermal radiation light and the normalization signal ofthe plasma light.

According to a fifth aspect of the fourth aspect of the disclosure,there is provided the laser welding quality inspection method, in whichin the calculating a normalization signal of the thermal radiation lightand a normalization signal of the plasma light, an average value m_(av)of the signal intensity of the thermal radiation light within thedetermination period, an average value n_(av) of the signal intensity ofthe plasma light within the determination period, a time function H(t)of the signal intensity of the thermal radiation light before beingnormalized within the determination period, a time function S(t) of thesignal intensity of the plasma light before being normalized within thedetermination period, a time function Hm(t) of the normalization signalof the thermal radiation light within the determination period, and atime function Sn(t) of the normalization signal of the plasma lightwithin the determination period respectively satisfy the followingexpressions.

Hm(t)=(H(t)−m _(av))/m _(av)  [Equation 1]

Sn(t)=(S(t)−n _(av))/n _(av)  [Equation 2]

According to a sixth aspect of the fourth aspect of the disclosure,there is provided the laser welding quality inspection method, in whichin the calculating a normalization signal of the thermal radiation lightand a normalization signal of the plasma light, an average valuem_(av)(t) of a time function of the signal intensity of the thermalradiation light of a plurality of times of welding determined to have noabnormality in the welded portion within the determination period, anaverage value n_(av)(t) of a time function of the signal intensity ofthe plasma light of the plurality of times of welding within thedetermination period, a time function H(t) of the signal intensity ofthe thermal radiation light before being normalized within thedetermination period, a time function. S(t) of the signal intensity ofthe plasma light before being normalized within the determinationperiod, a time function Hm(t) of the normalization signal of the thermalradiation light within the determination period, and a time functionSn(t) of the normalization signal of the plasma light within thedetermination period respectively satisfy the following expressions.

Hm(t)=(H(t)−m _(av)(t))/m _(av)(t)  [Equation 3]

Sn(t)=(S(t)−n _(av)(t))/n _(av)(t)  [Equation 4]

According to a seventh aspect of the disclosure, there is provided alaser welding quality inspection apparatus for a welded portion betweena joining object and a joined object, when the joining object and thejoined object are welded by being irradiated with a laser beam, theapparatus including: a measurement device and a welding statedetermination device, in which the welding state determination deviceincludes a signal intensity acquisitor that acquires, from themeasurement device, first data indicating a signal intensity of thermalradiation light radiated from the welded portion during welding, andsecond data indicating a signal intensity of plasma light radiated fromthe welded portion during the welding, and a signal intensity processorthat executes processing of the first data and the second data acquiredby the signal intensity acquisitor, and in which the signal intensityprocessor determines whether or not the welded portion includes anabnormality based on a comparison between the signal intensity of thethermal radiation light and the signal intensity of the plasma lightwhich are acquired.

According to an eighth aspect of the seventh aspect of the disclosure,there is provided the laser welding quality inspection apparatus, inwhich the signal intensity processor calculates a difference signalindicating a difference between the signal intensity of the thermalradiation light and the signal intensity of the plasma light based onthe first data and the second data, and determines that the weldedportion includes an abnormality when the calculated difference signalincludes a peak having a signal intensity larger than a presetdetermination reference value.

According to a ninth aspect of the eighth aspect of the disclosure,there is provided the laser welding quality inspection apparatus, inwhich the signal intensity acquisitor further acquires an irradiationoutput waveform indicating an intensity of irradiation light of thelaser beam from the measurement device, in which the signal intensityprocessor sets, as a determination period, a period during which theintensity of the irradiation light of the laser beam is constantlymaintained based on the irradiation output waveform, and extracts thesignal intensity of the thermal radiation light within the determinationperiod and the signal intensity of the plasma light within thedetermination period respectively from the signal intensity of thethermal radiation light and the signal intensity of the plasma light,and in which the difference signal indicates a difference between thesignal intensity of the thermal radiation light within the determinationperiod and the signal intensity of the plasma light within thedetermination period.

According to a tenth aspect of the ninth aspect of the disclosure, thereis provided the laser welding quality inspection apparatus, in which thesignal intensity processor calculates a normalization signal of thethermal radiation light and a normalization signal of the plasma lightby respectively normalizing the signal intensity of the thermalradiation light within the determination period and the signal intensityof the plasma light within the determination period, and in which thedifference signal indicates a difference between the normalizationsignal of the thermal radiation light and the normalization signal ofthe plasma light.

According to an eleventh aspect of the first aspect of the disclosure,there is provided the laser welding quality inspection method, in whichthe signal intensity of the thermal radiation light includes a firstthermal radiation light intensity indicating an intensity of the thermalradiation light at a first time point during the welding, and a secondthermal radiation light intensity indicating an intensity of the thermalradiation light at a second time point different from the first timepoint during the welding, in which the signal intensity of the plasmalight includes a first plasma light intensity indicating an intensity ofthe plasma light at the first time point and a second plasma lightintensity indicating an intensity of the plasma light at the second timepoint, and in which the comparison between the signal intensity of thethermal radiation light and the signal intensity of the plasma lightincludes calculating a first difference value indicating a differencebetween the first thermal radiation light intensity and the first plasmalight intensity, and a second difference value indicating a differencebetween the second thermal radiation light intensity and the secondplasma light intensity, and generating a difference signal including thefirst difference value and the second difference value.

According to a twelfth aspect of the disclosure, there is provided alaser welding quality inspection apparatus including: a processor; and amemory storing a program, in which when the program is executed, theprocessor performs, acquiring, from a first sensor, first dataindicating a signal intensity of thermal radiation light radiated from aworkpiece that has received a laser beam during laser welding,acquiring, from a second sensor, second data indicating a signalintensity of plasma light radiated from the workpiece during the laserwelding, and determining whether or not a welded portion of theworkpiece includes an abnormality based on a comparison between thesignal intensity of the thermal radiation light and the signal intensityof the plasma light.

Hereinafter, exemplary embodiments of the disclosure will be describedwith reference to the drawings. The disclosure is not limited to theexemplary embodiments below. Appropriate changes can be made withoutdeparting from the scope of the effect of the disclosure. Combinationswith other exemplary embodiments are possible.

Each drawing is a schematic view and is not necessarily strictlyillustrated.

In each drawing, the substantially same configurations are denoted bythe same reference numerals, and overlapping description will be omittedor simplified.

Exemplary Embodiments

First, an overall configuration of the laser welding quality inspectionapparatus according to the exemplary embodiment of the disclosure willbe described.

FIG. 1 is an overall view illustrating a configuration of the laserwelding quality inspection apparatus according to an exemplaryembodiment of the disclosure.

Laser welding quality inspection apparatus 100 illustrated in FIG. 1includes measurement device 20 and welding state determination device30. Measurement device 20 includes laser oscillator 1, collimator lens2, condenser lens 3, total reflection mirror 4, dichroic mirror 5, andlight receiving sensors 6, 7, and 8. Laser beam 10 emitted from laseroscillator 1 becomes a parallel beam through collimator lens 2, isreflected by total reflection mirror 4, and is condensed by condenserlens 3, and joined object 15 is irradiated with laser beam 10. Joiningobject 16 is installed below joined object 15. Joined object 15 andjoining object 16 are fixed on stage 17, moved by stage 17, andirradiated with laser beam 10 to be laser-welded.

During the laser welding, welding light 11 generated from joined object15 passes through condenser lens 3 and total reflection mirror 4, and iswavelength-separated by dichroic mirror 5. The wavelength-separatedwelding light is split into, for example, thermal radiation light (forexample, wavelength 1300 nm) 12 and plasma light (for example,wavelength 400 to 700 nm) 13 by a bandpass filter (not illustrated), andeach of them is incident on each of light receiving sensors 6 and 7.

On the other hand, laser beam 10 is not completely reflected by totalreflection mirror 4, and light of approximately 0.5% of a laser outputis transmitted through total reflection mirror 4, and transmitted laserbeam 14 is incident on light receiving sensor 8.

Three types of optical signals incident on light receiving sensors 6, 7,and 8 are transmitted to welding state determination device 30 andsubjected to a signal process. Welding state determination device 30determines whether or not the welded portion between joined object 15and joining object 16 includes an abnormality based on a result of thesignal process, and performs a quality inspection of the laser welding.In FIG. 1, laser beam 10 and welding light 11 are separately illustratedbetween total reflection mirror 4 and joined object 15, but in reality,laser beam 10 and welding light 11 pass through condenser lens 3 along asame path.

A welding state during normal welding according to the exemplaryembodiment of the disclosure will be described with reference toschematic views illustrated in FIGS. 2A and 2B. FIG. 2A is a sectionalview of the welded portion at a time of a start of the laser welding,and FIG. 2B is a sectional view of a welding state at a time of acompletion of the welding.

First, laser beam 10 condensed by condenser lens 3 is irradiated tojoined object 15 placed on joining object 16 (FIG. 2A). In the presentexemplary embodiment, for example, joining object 16 is an aluminummaterial having a thickness of 0.5 min, joined object 15 is also asimilar aluminum material having a thickness of 0.1 mm.

Next, after the welding is started, laser beam 10 is moved (from left toright as indicated by arrow P in FIG. 2B), on a straight line, relativeto joined object 15 by movement (from right to left, not illustrated) ofstage 17 supporting joining object 16 and joined object 15.

As illustrated in FIG. 2B, when the welding is completed, melted andsolidified portion 18 having a constant depth is formed in a scanningregion of laser beam 10. For example, a laser output at this time is 250W, a moving speed of stage 17 is 500 mm/s, and the depth of melted andsolidified portion 18 is approximately 200 μm.

FIG. 3 illustrates setting of the output waveform of the laser beamaccording to the present exemplary embodiment. As illustrated in FIG. 3,the output of the laser beam is set to a trapezoidal waveform andincludes slow-up portion (0 ms to T1), flat portion (T1 to T2), andslow-down portion (T2 to 4 ms), and a total irradiation time is 4 ins.The slow-up portion and the slow-down portion in the output waveform ofthe laser beam are provided to prevent spatter or depression during thelaser welding. In the welding by irradiation with the laser beam havingsuch a trapezoidal waveform, the shape of melted and solidified portion18 is formed into an inverted trapezoidal shape (FIG. 2B).

Next, the welding state during abnormal welding according to theexemplary embodiment of the disclosure will be described by usingschematic views illustrated in FIGS. 4A and 4B. FIG. 4A is a sectionalview of a welded portion at a time of a start of the laser welding, andFIG. 4B is a sectional view of a welding state at a time of a completionof the welding.

In the laser welding, especially when a thin plate having a thickness of0.1 mm is welded, an occurrence of perforation or spatter due to aforeign matter sandwiched at a joining interface between the joiningobject and the joined object occupies a majority of abnormal weldingcases. Therefore, the welding state when the foreign matter issandwiched at the joining interface is schematically illustrated inFIGS. 4A and 4B.

As illustrated in FIG. 4A, at the time of the start of the welding,joined object 15 placed on joining object 16 is irradiated with laserbeam 10. At this time, for example, resin foreign matter 21 issandwiched at the joining interface between joining object 16 and joinedobject 15.

Next, after the start of the welding, laser beam 10 is moved (from leftto right as indicated by arrow P in FIG. 4A) on a straight line relativeto joined object 15 by the movement (from right to left, notillustrated) of stage 17 supporting joining object 16 and joined object15. When the laser welding progresses with the movement of laser beam 10and laser beam 10 hits resin foreign matter 21, resin foreign matter 21is rapidly sublimated by the irradiation with laser beam 10. Therefore,in melted and solidified portion 18, the molten material (aluminum)around resin foreign matter 21 is blown off, so that any one or aplurality of perforations 22, abnormal projections 23, and spatters 24are generated (see FIG. 4B).

When perforation 22 occurs, the joining intensity decreases, whenabnormal projection 23 occurs, the external appearance becomes poor, andwhen spatter 24 occurs, the foreign matter is mixed into an inside ofthe product. Therefore, the abnormality occurred in the welded portioncauses various product defects. Therefore, it is necessary to detectsuch welding abnormalities in real time and eliminate defective weldedproducts.

Next, a difference in welding light generated during normal welding andabnormal welding according to the present exemplary embodiment will bedescribed below.

First, a state during the normal welding will be described.

FIG. 5A illustrates the external appearance of melted and solidifiedportion 18 during the normal welding according to the present exemplaryembodiment. An upper part of FIG. 5B illustrates an irradiation waveformof the laser beam obtained by light receiving sensor 8, in which ahorizontal axis represents an irradiation time and a vertical axisrepresents a signal intensity. A lower part of FIG. 5B illustrates thesignal intensity of the thermal radiation light obtained by lightreceiving sensor 6, in which a horizontal axis represents an irradiationtime and a vertical axis represents a signal intensity of the thermalradiation light. The signal intensity represented on these vertical axesis proportional to intensity (W) of the light incident on each of lightreceiving sensors 8 and 6, and is expressed by using any unit(a.u.=arbitrary unit). The output waveform of the laser beam is set inthe trapezoidal shape as illustrated in FIG. 3.

When the output waveform of the trapezoidal laser beam is set, theoutput waveform of the laser beam with which joining object 16 andjoined object 15 are actually irradiated also has a substantiallytrapezoidal shape as illustrated in FIG. 5B.

In other words, the irradiation waveform of the laser beam has a shapeaccording to the setting of the output waveform of the laser beam, andhas a flat portion in irradiation period a1 to a2 so as to correspond toflat portion (T1 to T2, FIG. 3) in an output design of the trapezoidallaser beam.

Since the thermal radiation light generated during the welding basicallyhas a signal intensity corresponding to the irradiation output of thelaser beam, as illustrated in FIG. 5B, it becomes a substantiallytrapezoidal shape having the flat portion in the irradiation period a1to a2.

During the normal welding, for example, an external shape of melted andsolidified portion 18 illustrated in FIG. 5A has a melting length ofapproximately 2 mm and a melting width of approximately 0.2 mm.

Next, a state during the abnormal welding will be described.

FIG. 6A illustrates an external appearance of melted and solidifiedportion 18 during the abnormal welding according to the presentexemplary embodiment. During the abnormal welding, perforation 25 isseen in a central portion of melted and solidified portion 18. Duringthe welding, it is considered that at this location, the resin foreignmatter at the joining interface is rapidly sublimated by the irradiationof the laser beam, and a molten material is blown off, wherebyperforation 25 is formed. The signal intensity of the thermal radiationlight obtained at this time is illustrated in a lower part of FIG. 6B.As clearly illustrated by a lower graph of FIG. 6B, abnormal peak 26appears in the signal intensity of the thermal radiation light when theabnormal welding occurs.

Abnormal peak 26 is specifically a peak having a very large signalintensity. This is due to abnormal heat generation of the resin foreignmatter, blowout of the molten material, and the like.

That is, during the normal welding, the signal intensity of the thermalradiation light in irradiation period a1 to a2 corresponding to the flatportion of the irradiation output waveform of the laser beam illustratedin the upper part of FIG. 5B has a relatively uniform distributionwithout having the abnormal peak as illustrated in the lower part ofFIG. 5B. On the other hand, during the abnormal welding, in the signalintensity of the thermal radiation light in irradiation period b1 to b2corresponding to the flat portion of the irradiation output waveform ofthe laser beam illustrated in the upper part of FIG. 6B, abnormal peak26 appears corresponding to the time of occurrence of the abnormalwelding as illustrated in the lower part of FIG. 6B. In the weldingdefect determination of the related art, for example, determination ofthe welding defect is performed by determining whether or not a peakhaving a signal intensity exceeding a certain determination referencevalue is included in the detected signal intensity of the thermalradiation light based on presence or absence of this abnormal peak.

The accuracy of the welding abnormality determination based on the peakintensity of thermal radiation light according to the related art wasverified as follows.

<<Verification of Welding State Determination by Related Art>>

Verification of the welding state determination by the related art wasperformed by using a total of 89 welded products, 51 normal weldedproducts, and 38 abnormal welded products. Here, the normal weldedproduct is, specifically, a welded product in which no foreign matter ismixed in the joining interface, and the abnormal welded product is,specifically, a welded product in which the foreign matter is mixed inthe joining interface. FIG. 7A and FIG. 7B illustrate the thermalradiation light during welding of welded products having a plurality ofdifferent welding states in the welding defect determination of therelated art. FIG. 7A illustrates signal intensities of peaks in thethermal radiation light of a plurality of welded products. A horizontalaxis represents the number of perforations generated in the melted andsolidified portion, and a vertical axis represents the signal intensityof the peak in the thermal radiation light. Even if one perforation isformed, the welding is defective, and if two perforations are formed,the state of the welding defect is further deteriorated. In order tomore clearly illustrate the determination of normality/abnormality, FIG.7B is an enlarged view of a portion where the signal intensities of thepeaks in the thermal radiation light of the plurality of welded productsillustrated in FIG. 7A are equal to or less than 700. In this case, asillustrated in FIG. 7B, the signal intensities of the thermal radiationlight generated during the welding of the welded products having 0 or 1perforation are illustrated.

As illustrated in FIGS. 7A and 7B, the signal intensities of the thermalradiation light generated during the welding of the normal weldedproducts are approximately 320 to 530, whereas the signal intensities ofthe thermal radiation light generated during the welding of the abnormalwelded products having one perforation are approximately 430 to 2400,and the signal intensities of the thermal radiation light generatedduring the welding of the abnormal welded products having twoperforations are approximately 900 to 4100.

In this case, for example, in a case where 530, which is a maximum valueof the signal intensity of the thermal radiation light generated duringthe welding of the normal welded product, is used as a determinationreference value of the welding defect, all of the abnormal weldedproducts, in which two perforations are formed, can be determined as thewelding abnormalities. However, all the abnormal welded products havingone perforation are not determined as the welding abnormalities, anderroneous determinations occur. Specifically, among the abnormal weldedproducts having one perforation, in five abnormal welded products inwhich the signal intensity of the thermal radiation light isapproximately 430 to 500, if the determination is performed based on theabove-described determination reference value, the presence of theabnormal peak is not determined, and the welding is determined asnormal. That is, in a determination result of the welding defect of therelated art illustrated in FIG. 8, among a total of 38 abnormal weldedproducts, 5 erroneous determinations occur, a correct determination rateis 87%, and an erroneous determination rate is 13%.

On the other hand, if it is attempted to determine 5 abnormal weldedproducts as welding abnormalities, it is necessary to set adetermination threshold to equal to or less than 430. In this case, alarge number of normal welded products are determined as weldingabnormalities, thereby resulting in the erroneous determination (seeFIG. 7B).

As described above, in the welding defect determination method of therelated art, it is necessary to improve the accuracy of the weldingdefect determination. Therefore, the present inventors analyzed thecause of the erroneous determination in the welding defect determinationof the related art as follows.

FIG. 9A illustrates an external appearance of the melted and solidifiedportion of the abnormal welded product in the welding defectdetermination of the related art. FIG. 9B illustrates the signalintensity of the thermal radiation light during the welding of theabnormal welded product in the welding defect determination of therelated art. As illustrated in FIG. 9A, a minute perforation isgenerated in a central portion of the melted and solidified portion asindicated by arrow A. Correspondingly, in the signal intensity of thethermal radiation light during the welding illustrated in FIG. 9B, smallpeak 27 appears corresponding to the occurrence of abnormal meltingillustrated by arrow B. The signal intensity of the thermal radiationlight illustrated in peak 27 is approximately 500.

FIG. 10A illustrates the external appearance of the melted andsolidified portion of the normal welded product in the welding defectdetermination of the related art. FIG. 10B is a graph illustrating thesignal intensity of the thermal radiation light during the welding ofthe normal welded product in the welding defect determination of therelated art. As illustrated in FIG. 10B, in the normal welded product,the signal intensity of the thermal radiation light generated during thewelding fluctuates within a range of approximately 300 to 500. That is,in the signal intensity of the thermal radiation light measured for theabnormal welded product illustrated in FIG. 9B, the signal intensity ofpeak 27 by the occurrence of the abnormal welding is within thefluctuation range of the signal intensity of the thermal radiation lightmeasured for the normal welded product illustrated in FIG. 10B.

From the above analysis, it is clear that in the laser welding, abehavior of the signal intensity of thermal radiation light by theminute welding abnormalities in some abnormal welded products cannot bedistinguished from the fluctuation of the signal intensity of thethermal radiation light in the normal welded products. Such a phenomenonoccurs because, in a case where an influence of the minute weldingabnormality on the thermal radiation light is small, the influence isburied in the fluctuation of the welding light generated during thewelding. Therefore, as illustrated in FIGS. 7A and 7B, the measuredsignal intensities of the peaks in the thermal radiation light duringthe welding are overlapped with each other between the normal weldedproduct and the abnormal welded product. As a result, as describedabove, an appropriate determination reference value cannot be set,thereby resulting in an erroneous determination. Therefore, the weldingdefect determination by the peak intensity of the thermal radiationlight of the related art has a problem that the minute weldingabnormality cannot be accurately determined.

Therefore, the present inventors obtained the following new findings asa result of repeated studies in order to detect, with higher accuracy,the occurrence of the welding abnormality in the laser welding.

The present inventors also measured the plasma light generated duringthe welding at the same time as the measurement of the thermal radiationlight, and paid attention to a change in the signal intensities of both.FIG. 11 illustrates the measurement results of the signal intensity ofthe thermal radiation light and the signal intensity of the plasma lightgenerated during the laser welding according to the present exemplaryembodiment. (a) of FIG. 11 illustrates the signal intensity (solid line)of the thermal radiation light and the signal intensity (dotted line) ofthe plasma light of the normal welded product. (b) of FIG. 11illustrates the signal intensity (solid line) of the thermal radiationlight and the signal intensity (dotted line) of the plasma light ofabnormal welded product 1 determined as the welding abnormality in themethod of the related art. (c) of FIG. 11 illustrates the signalintensity (solid line) of the thermal radiation light and the signalintensity (dotted line) of the plasma light of abnormal welded product 2that was not determined as the welding abnormality in the method of therelated art.

As illustrated in (a) of FIG. 11, during the normal welding, inirradiation period c1 to c2 of the laser beam, the thermal radiationlight and the plasma light generated in the welded portion illustratesimilar behaviors, and have similar signal intensities to each other.

On the other hand, in the abnormal welded product of (b) of FIG. 11,both the thermal radiation light and the plasma light have steep peaksat a time point at which the welding abnormality occurs (indicated byarrow C).

Here, what the present inventors noticed is the signal intensity of thepeak generated in the thermal radiation light and the plasma lightduring the abnormal welding. As illustrated in (b) of FIG. 11, thesignal intensity of the peak generated during the abnormal welding ismuch larger in the thermal radiation light than that in the plasmalight. The signal intensity of the thermal radiation light changesgreatly when the welding abnormality occurs by the resin foreign matterat the joining interface, because the temperature rises sharply due tothe rapid sublimation of the resin foreign matter, and a peak having alarge signal intensity appears. On the other hand, when the weldingabnormality occurs, the plasma light does not change so much as comparedwith the thermal radiation light. Therefore, when the weldingabnormality occurs, a difference occurs in the change in signalintensity between the thermal radiation light and the plasma light.

In abnormal welded product 2 illustrated in (c) of FIG. 11, a small peakappears in the signal intensity of the thermal radiation light at thetime point (indicated by arrow D) at which the welding abnormalityoccurs. On the other hand, no peak appears in the signal intensity ofthe plasma light. This is because, as described above, the thermalradiation light is more affected by the welding abnormality than theplasma light, even if there is a minute welding abnormality, it isconsidered that the change in the thermal radiation light appears, butthe change in the plasma light does not appear. That is, as in the caseof (b) of FIG. 11, when the welding abnormality occurs, there is adifference in the change in the signal intensity between the thermalradiation light and the plasma light.

The present inventors found that the thermal radiation light and theplasma light during the normal welding exhibit similar behaviors to eachother, whereas the thermal radiation light and the plasma light duringthe abnormal welding have a difference in the change in the signalintensity; based on this, by evaluating the difference signal betweenthe thermal radiation light and the plasma light, the abnormal weldingcan be accurately determined even for the minute welding abnormality.Based on this new finding, the present inventors developed a laserwelding quality inspection method and apparatus according to thedisclosure.

FIG. 12 is a block diagram illustrating a configuration of welding statedetermination device 30 in laser welding quality inspection apparatus100 of FIG. 1.

As illustrated in FIG. 12, welding state determination device 30includes signal intensity acquisitor 31, signal intensity processor 32,storage 33, and output 34, and is electrically connected to measurementdevice 20. Signal intensity acquisitor 31 acquires, from measurementdevice 20, first data indicating the signal intensity of the thermalradiation light radiated from the welded portion and second dataindicating the signal intensity of the plasma light during the welding.Signal intensity processor 32 determines whether or not an abnormalityoccurs during the welding by executing the processing of the first dataand the second data acquired by signal intensity acquisitor 31. Storage33 may be, for example, an auxiliary storage device such as a hard diskdrive, and stores a data processing program executed by signal intensityprocessor 32, various data, and the like. Output 34 may be an outputinterface circuit that outputs data from welding state determinationdevice 30 to the outside. Welding state determination device 30 may be,for example, a computer including a processor and storage 33 that storesa program. When executing the program, the processor executes a dataprocess. Specifically, the processor acquires, from light receivingsensor 7, first data indicating the signal intensity of the thermalradiation light radiated from the workpiece that received the laser beamduring the laser welding. The processor further acquires, from lightreceiving sensor 7, second data indicating the signal intensity of theplasma light radiated from the workpiece during the laser welding. Theprocessor further determines whether or not the welded portion of theworkpiece includes an abnormality based on a comparison between thesignal intensity of the thermal radiation light and the signal intensityof the plasma light.

Welding state determination device 30 may also acquire the dataprocessing program executed by signal intensity processor 32 from aportable storage medium. The storage medium is a medium accumulatinginformation such as a program by an electric, magnetic, optical,mechanical, or chemical action so that the computer, another device,machine, or the like can read the information such as the recordedprogram.

<<Determination Process of Welding State>>

FIG. 13 is a flowchart of a determination process of a welding state oflaser welding quality inspection apparatus 100 according to theexemplary embodiment of the disclosure. The determination process of thewelding state of the laser welding will be described with reference toFIG. 13.

(1) First, in step S101, laser welding is started.

(2) Next, in step S102, at the same time as the laser welding,measurement device 20 simultaneously performs irradiation outputmeasurement of the laser beam 1021, measurement of the thermal radiationlight 1022, and measurement of the plasma light 1023. Signal intensityacquisitor 31 acquires, from measurement device 20, data of theirradiation output waveform of the laser beam, the signal intensity ofthe thermal radiation light, and the signal intensity of the plasmalight.

(3) In step S103, signal intensity processor 32 determines thedetermination period based on the acquired irradiation output waveformof the laser beam. Signal intensity processor 32 further extracts, fromthe measurement data of the thermal radiation light and the plasmalight, data of the signal intensity of the thermal radiation light anddata of the signal intensity of the plasma light within thedetermination period, respectively (1031 and 1032). A specific dataextraction method will be described later.

(4) Subsequently, in step S104, signal intensity processor 32 performssignal normalization on the data of the extracted signal intensity ofthe thermal radiation light and the extracted signal intensity of theplasma light, respectively (1041 and 1042). A specific signalnormalization method will be described later.

(5) Subsequently, in step S105, signal intensity processor 32 subtractsthe normalized signal intensity of the plasma light from the normalizedsignal intensity of the thermal radiation light to calculate adifference signal indicating the difference between the normalizedsignal intensity of the thermal radiation light and the normalizedsignal intensity of the plasma light. A specific calculation method ofthe difference signal will be described later.

(6) Next, in step S106, signal intensity processor 32 determines whetheror not an abnormality has occurred in the welded portion based on thesignal intensity of the peak of the difference signal.

(7) Finally, in step S107 and step S108, signal intensity processor 32determines the welding state based on the signal intensity of the peakof the difference signal. Specifically, when the calculated differencesignal includes a peak having a signal intensity larger than a certaindetermination reference value, it is determined as welding abnormality(step S107). On the other hand, when the calculated difference signaldoes not include a peak having a signal intensity larger than a certaindetermination reference value, it is determined as normal welding (stepS108).

After that, the determination result of the welding state by signalintensity processor 32 is output via output 34, and the processedproduct, which is determined as the welding abnormality based on thedetermination result, may be discharged as a defective product from theprocess. The processed product determined as the normal welding flows tothe next step as a good product, for example. The determinationreference value used for the determination may be determined by a basicexperiment and is stored in storage 33. The determination referencevalue may be changed depending on a material of the processed product,output setting of the laser beam, or the like.

<<Data Extraction Method>>

The data extraction step S103 of the determination process of thewelding states illustrated in FIG. 13 will be described in detail withreference to FIGS. 14 and 15. FIG. 14 illustrates the execution resultof step S102 in the determination flowchart of the welding state of FIG.13. (a) of FIG. 14 illustrates the irradiation output waveform of thelaser beam measured in step S102. (b) of FIG. 14 illustrates the signalintensity of the thermal radiation light of the welded product havingthe minute welding abnormality measured in step S102, and (c) of FIG. 14illustrates the signal intensity of the plasma light of the weldedproduct having the minute welding abnormality measured in step S102. Atthis time, as illustrated in (b) of FIG. 14, a small peak (indicated byarrow E) appears in the signal intensity of the thermal radiation lightwhen the minute abnormal welding occurs, and since the signal intensityof the peak is within the fluctuation range of the signal intensity ofthe thermal radiation light during the welding, abnormal welding is notdetermined by the welding defect determination of the related art by thepeak intensity of the thermal radiation light. The signal intensity ((c)of FIG. 14) of the plasma light during the welding fluctuates up anddown, and no peak appears when minute abnormal welding occurs.

As illustrated in (a) of FIG. 14, the irradiation output waveform of thelaser beam according to the present exemplary embodiment has asubstantially trapezoidal shape. Slow-up portion t1 in the trapezoidalwaveform is provided to prevent spatter at the time of the start of thewelding, and slow-down portion t3 is provided to prevent depression atthe end of the welding. Therefore, slow-up portion t1 and slow-downportion t3 are regions where the laser output is weak, and joined object15 and joining object 16 are not welded. That is, the welding of joinedobject 15 and joining object 16 is carried out in flat portion t2 of thetrapezoidal waveform. Therefore, in step S106 in FIG. 13, it ispreferable to determine the state of the welded portion by using thedifference signal between the thermal radiation light and the plasmalight corresponding to only flat portion t2 in the irradiation outputwaveform of the trapezoidal laser beam. Therefore, in step S103, theirradiation period corresponding to flat portion t2 of the trapezoidalwaveform of the laser beam having a constant irradiation output is setas the determination period, and the data of the signal intensity of thethermal radiation light and the data of the signal intensity of theplasma light within the determination period are respectively extracted.

Specifically, first, time ranges respectively corresponding to slow-upportion t1, flat portion t2, and slow-down portion t3 in the irradiationoutput waveform of the laser beam are specified. For example, in theirradiation output waveform of the laser beam illustrated in (a) of FIG.14, regions having three different slopes of positive, zero, andnegative are specified by, for example, obtaining an approximatestraight line. The time ranges corresponding to regions having therespective slopes are obtained. Here, the region having the positiveslope corresponds to slow-up portion t1, the region having the zeroslope corresponds to flat portion t2, and the region having the negativeslope corresponds to slow-down portion t3.

Next, the region having the zero slope, that is, the time range of flatportion t2 of the irradiation output waveform, in which the laser beamhas a constant irradiation output, is set as the determination period,and in the data of the signal intensity of the thermal radiation lightand the data of the signal intensity of the plasma light which areacquired, data of the signal intensity of the thermal radiation lightand data of the signal intensity of the plasma light within thedetermination period are extracted.

Setting of the output waveform of the laser beam illustrated in FIG. 3may be used to specify the time ranges of the slow-up portion, the flatportion, and the slow-down portion. In this case, since the trapezoidalshape is clearer than that in a case where the irradiation outputwaveform of the laser beam is used, it is possible to more easilyspecify the time range (section T1 to T2 in FIG. 3) corresponding to theflat portion. On the other hand, the irradiation output waveform of thelaser beam is the output of the irradiation light during actual laserwelding. Therefore, by using the irradiation output waveform of thelaser beam, it is possible to more accurately specify the determinationperiod and extract the data. This is because the irradiation outputwaveform of the laser beam that is actually output is affected by anoptical design or the like, and does not necessarily match a presetoutput waveform of the laser beam.

The result of executing step S103 is illustrated in FIG. 15. (a) of FIG.15 illustrates the signal intensity of the thermal radiation light inextracted determination period t2, and (b) of FIG. 15 illustrates thesignal intensity of the plasma light in extracted determination periodt2.

<<Signal Normalization Method>>

Next, the signal normalization in step S104 of the determination processof the welding states illustrated in FIG. 13 will be described in detailwith reference to FIG. 16. In order to obtain the difference signalbetween the thermal radiation light and the plasma light, it isnecessary to match respective absolute intensity levels. The signalintensity of the thermal radiation light depends on a sensitivity and asize of the light receiving sensor, light attenuation by an opticalsystem until reaching the light receiving sensor, an amplificationfactor in the signal processing circuit, or the like. The same appliesto the plasma light. Therefore, preferably, in step S104 in FIG. 13, thesignal intensities of the thermal radiation light and the plasma lightextracted in step S103 are normalized.

The signal normalization method will be specifically described below.

First, for the welded product of 1, an average value m_(av) of thesignal intensity of the thermal radiation light within determinationperiod t2 and an average value n_(av) of the signal intensity of theplasma light within determination period t2 are calculated. Next, adifference between a time function H(t) of the signal intensity of thethermal radiation light within determination period t2 and the averagevalue m_(av), and a difference between a time function. S(t) of thesignal intensity of the plasma light within determination period t2 andthe average value n_(av) are calculated, and further divided by theaverage values m_(av) and n_(av), respectively, to obtain normalizationsignals Hm1(t) and Sn1(t) of the time function of the thermal radiationlight and the time function of the plasma light within determinationperiod t2. That is, the normalization signals Hm1(t) and Sn1(t) satisfythe following expressions, respectively.

Hm1(t)=(H(t)−m _(av))/m _(av)  [Equation 5]

Sn1(t)=(S(t)−n _(av))/n _(av)  [Equation 6]

In step S104 of the determination process of the welding statesillustrated in FIG. 13, signal intensity processor 32 executes the abovecalculation on each of the signal intensity of the thermal radiationlight and the signal intensity of the plasma light acquired bymeasurement device 20. Then, the normalization signal of the thermalradiation light within determination period t2 and the normalizationsignal of the plasma light within determination period t2 are obtained.

The normalization of the signal intensity of the thermal radiation lightand the signal intensity of the plasma light can be performed by usingdifferent methods. Other signal normalization methods will be describedin detail below.

First, in advance, as a reference value, time function m_(av)(t) of anaverage signal intensity of the thermal radiation light of a pluralityof normal welded products and time function n_(av)(t) of an averagesignal intensity of the plasma light are calculated. Next, a differencebetween the time function H(t) of the signal intensity of the thermalradiation light within determination period t2 and a time functionm_(av)(t) of the average signal intensity of the thermal radiation lightof a reference value, and a difference between the time function S(t) ofthe signal intensity of the plasma light within determination period t2and a time function n_(av)(t) of the average signal intensity of theplasma light of a reference value are calculated, and further divided bythe time function m_(av)(t) of the average signal intensity of thethermal radiation light and the time function n_(av)(t) of the averagesignal intensity of the plasma light respectively, to obtain thenormalization signals Hm2(t) and Sn2(t) of the time function of thethermal radiation light and the time function of the plasma light withindetermination period t2. That is, the normalization signals Hm2(t) andSn2(t) satisfy the following expressions, respectively.

Hm2(t)=(H(t)−m _(av)(t))/m _(av)(t)  [Equation 7]

Sn2(t)=(S(t)−n _(av)(t))/n _(av)(t)  [Equation 8]

Such normalization signals Hm2(t) and Sn2(t) are particularly useful,for example, in a case where a portion except for the slow-up portionand slow-down portion is not flat in the output waveform of the setlaser beam. This is because the normalization method can performnormalization including the vertical fluctuation of the output intensityin the output waveform of the laser beam included in the portionexcluding the slow-up portion and the slow-down portion.

The result of executing step S104 is illustrated in FIG. 16. (a) of FIG.16 is the normalization signal of the thermal radiation light withindetermination period t2, and (b) of FIG. 16 is the normalization signalof the plasma light in determination period t2.

<<Calculation Method of Difference Signal>>

Next, the calculation of the difference signal in step S105 of thedetermination process of the welding state illustrated in FIG. 13 willbe described in detail with reference to FIGS. 17A to 17C. In step S105,the difference signal in determination period t2 is calculated byobtaining the difference between the normalization signal of the thermalradiation light and the normalization signal of the plasma lightobtained in step S104.

Here, the difference signal is preferably calculated as an absolutevalue. By calculating the absolute value of the difference signal, whenthe signal intensity of the peak of the difference signal, which will bedescribed later, is determined, the determination reference value may beset to a positive value and therefore the setting becomes simple. In acase where the absolute value of the difference signal is notcalculated, when the difference signal is calculated by subtracting thesignal intensity of plasma light from the signal intensity of thermalradiation light, the determination reference value is set to a positivevalue. When the difference signal is calculated by subtracting thesignal intensity of the thermal radiation light from the signalintensity of the plasma light, the determination reference value is setto a negative value.

The signal intensity of the thermal radiation light includes a firstthermal radiation light intensity (for example, −0.05) indicating theintensity of the thermal radiation light at a first time point (forexample, an irradiation time of 2.0 ms) during the welding, and a secondthermal radiation light intensity (for example, 0.3) indicating theintensity of the thermal radiation light at a second time point (forexample, an irradiation time of 2.2 ms) during the welding. The signalintensity of the plasma light includes a first plasma light intensity(for example, −0.05) indicating the intensity of the plasma light at afirst time point (for example, an irradiation time of 2.0 ms) and asecond plasma light intensity (for example, −0.2) indicating theintensity of the plasma light at a second time point (for example, anirradiation time of 2.2 ms). Signal intensity processor 32 may calculatea first difference value (for example, 0) indicating a differencebetween the first thermal radiation light intensity and the first plasmalight intensity, and a second difference value (for example, 0.5)indicating a difference between the second thermal radiation lightintensity and the second plasma light intensity to generate a differencesignal including the first difference value and the second differencevalue.

By executing step S105 of the determination process of the welding stateillustrated in FIG. 13, for the thermal radiation light ((b) of FIG. 14)and the plasma light ((c) of FIG. 14) in the determination period of thewelded product having the minute welding abnormality, a result ofcalculating the difference signal in determination period t2 isillustrated in FIG. 17A. The determination of step S106 in FIG. 13 isperformed based on the difference signal.

In step S106 of the determination process of the welding state, forexample, the determination reference value is set to 0.35 (illustratedby broken line in of FIG. 17B), and determination of the welded producthaving the minute welding abnormality by the calculated differencesignal is illustrated in FIG. 17B. As illustrated in FIG. 17B, since thedifference signal has a peak (indicated by arrow F) having a signalintensity larger than the determination reference value, it isdetermined as abnormal welding. That is, for the welded product havingthe minute welding abnormality; in which the occurrence of the abnormalwelding is not determined by the welding defect determination by thepeak intensity of the thermal radiation light of the related art ((b) ofFIG. 14), the abnormal welding can be accurately determined by thedetermination by the difference signal according to the disclosure.

On the other hand, FIG. 17C illustrates the determination of the normalwelded product by the calculated difference signal. The determinationreference value is also similarly set to 0.35 (indicated by broken linen of FIG. 17C). In this case, since there is no peak having a signalintensity larger than the determination reference value in thedifference signal, it is determined as normal welding.

<<Verification of Welding State Determination According to Disclosure>>

By using the determination method of the welding state according to thedisclosure described above, as in FIG. 7A and FIG. 7B, verification ofthe welding state determination was performed by using a total of 89welded products, 51 normal welded products, and 38 abnormal weldedproducts. FIG. 18A and FIG. 18B illustrate difference signals betweenthe normalization signal of the thermal radiation light and thenormalization signal of the plasma light in the determination period ofthe welded product having a plurality of different welding states in thedetermination of the welding state according to the disclosure. FIG. 18Aillustrates signal intensities of peaks in difference signals of aplurality of welded products. A horizontal axis represents the number ofperforations generated in the melted and solidified portion, and avertical axis represents the signal intensity of the peak in thedifference signal of the determination period. Even if one perforationis generated, welding defect occurs and if two perforations aregenerated, the welding defect state becomes worse. In order to moreclearly illustrate the determination of normality/abnormality, FIG. 18Bis an enlarged view of a portion where the signal intensity of the peakin the difference signals of the plurality of welded productsillustrated in FIG. 18A is equal to or less than 1.0.

As illustrated in FIGS. 18A and 18B, in the normal welded products, thesignal intensities of the peaks of the difference signals within thedetermination period are all equal to or less than approximately 0.3. Onthe other hand, in the abnormal welded products having 1 to 2perforations, the signal intensities of the peaks in the differencesignals within the determination period are all equal to or more thanapproximately 0.4.

Here, for example, in a case where the determination reference value is0.35 (indicated by broken line s in FIG. 18B) that is the signalintensity of the peak of the difference signal, all 51 normal weldedproducts are determined as normal welding, and all 38 abnormal weldedproducts having 1 to 2 perforations are determined as abnormal welding.That is, in the determination result of the welding state according tothe present exemplary embodiment illustrated in FIG. 19, among a total38 abnormal welded products, an erroneous determination does not occur,the erroneous determination rate is 0%, and the correct determinationrate is 100%. Therefore, the welding state determination methodaccording to the disclosure can determine the welding abnormality withhigher accuracy.

The disclosure is not limited to the exemplary embodiments describedabove, and can be implemented in various other modes. For example, inthe above description, the overlapping laser process is described as anexample, but the disclosure is not limited to this. Similar effects canbe obtained even when processing with other types of lasers is used.

While the disclosure is fully described in connection with the preferredexemplary embodiments with reference to the accompanying drawings,various variations and modifications will be apparent to those skilledin the art. It is to be understood that such variations andmodifications are included within the scope of the disclosure as long asthey do not depart from the scope of the disclosure as defined by theappended claims.

For example, signal intensity processor 32 determines whether or not thewelded portion includes an abnormality based on the signal intensity ofthe thermal radiation light during determination period t2 and thesignal intensity of the plasma light during determination period t2, butthe disclosure is not limited to this. Signal intensity processor 32 maydetermine whether or not the welded portion includes an abnormalitybased on the signal intensity of the thermal radiation light duringentire period (t1 to t3) and the signal intensity of the plasma lightduring entire period (t1 to t3).

Signal intensity processor 32 determines whether or not the weldedportion includes an abnormality based on the normalized signal intensityof the thermal radiation light and the normalized signal intensity ofthe plasma light, but the disclosure is not limited to this. Signalintensity processor 32 may determine whether or not the welded portionincludes an abnormality based on an unnormalized signal intensity of thethermal radiation light and an unnormalized signal intensity of theplasma light.

Signal intensity processor 32 determines whether or not the weldedportion includes an abnormality based on the difference signalindicating the difference between the signal intensity of the thermalradiation light and the signal intensity of the plasma light, but thedisclosure is not limited to this. Signal intensity processor 32 maydetermine whether or not the welded portion includes an abnormalitybased on a ratio signal indicating a ratio of the signal intensity ofthe thermal radiation light and the signal intensity of the plasmalight.

The laser welding quality inspection method and the laser weldingquality inspection apparatus of the disclosure can accurately determineeven with the minute welding abnormality by the signal intensity of thepeak in the difference signal between the thermal radiation light andthe plasma light generated during the welding, and can prevent theabnormal welded product from flowing out to the subsequent process ofthe laser welding process.

What is claimed is:
 1. A laser welding quality inspection method of awelded portion between a joining object and a joined object, when thejoining object and the joined object are welded by being irradiated witha laser beam, the method comprising: acquiring first data indicating asignal intensity of thermal radiation light radiated from the weldedportion during the welding; acquiring second data indicating a signalintensity of plasma light radiated from the welded portion during thewelding; and determining whether or not the welded portion includes anabnormality based on a comparison between the signal intensity of thethermal radiation light and the signal intensity of the plasma lightwhich are acquired.
 2. The laser welding quality inspection method ofclaim 1, wherein the determining whether or not the welded portionincludes an abnormality based on a comparison between the signalintensity of the thermal radiation light and the signal intensity of theplasma light which are acquired includes calculating a difference signalindicating a difference between the signal intensity of the thermalradiation light and the signal intensity of the plasma light, anddetermining that the welded portion includes an abnormality when thecalculated difference signal includes a peak having a signal intensitylarger than a preset determination reference value.
 3. The laser weldingquality inspection method of claim 2, further comprising: acquiring anirradiation output waveform indicating an intensity of irradiation lightof the laser beam by measuring the irradiation light of the laser beamduring the welding, wherein the calculating a difference signalindicating a difference between the signal intensity of the thermalradiation light and the signal intensity of the plasma light furtherincludes setting, as a determination period, a period during which theintensity of the irradiation light of the laser beam is constantlymaintained, based on the irradiation output waveform, and extracting thesignal intensity of the thermal radiation light within the determinationperiod and the signal intensity of the plasma light within thedetermination period respectively from the signal intensity of thethermal radiation light and the signal intensity of the plasma lightwhich are acquired, and wherein the calculating a difference signalindicating a difference between the signal intensity of the thermalradiation light and the signal intensity of the plasma light includescalculating a difference between an intensity of the thermal radiationlight within the determination period and an intensity of the plasmalight within the determination period.
 4. The laser welding qualityinspection method of claim 3, wherein the calculating a differencesignal indicating a difference between the signal intensity of thethermal radiation light within the determination period and the signalintensity of the plasma light within the determination period includescalculating a normalization signal of the thermal radiation light and anormalization signal of the plasma light by respectively normalizing thesignal intensity of the thermal radiation light within the determinationperiod and the signal intensity of the plasma light within thedetermination period, and calculating a difference signal indicating adifference between the normalization signal of the thermal radiationlight and the normalization signal of the plasma light.
 5. The laserwelding quality inspection method of claim 4, wherein in the calculatinga normalization signal of the thermal radiation light and anormalization signal of the plasma light, the following expressions aresatisfiedHm(t)=(H(t)−m _(av))/m _(av)  [Equation 1]:Sn(t)=(S(t)−n _(av))/n _(av)  [Equation 2]: where m_(av) is an averagevalue of the signal intensity of the thermal radiation light within thedetermination period, n_(av) is an average value of the signal intensityof the plasma light within the determination period, H(t) is a timefunction of the signal intensity of the thermal radiation light beforebeing normalized within the determination period, S(t) is a timefunction of the signal intensity of the plasma light before beingnormalized within the determination period, Hm(t) is a time function ofthe normalization signal of the thermal radiation light within thedetermination period, and Sn(t) is a time function of the normalizationsignal of the plasma light within the determination period.
 6. The laserwelding quality inspection method of claim 4, wherein in the calculatinga normalization signal of the thermal radiation light and anormalization signal of the plasma light, the following expressions aresatisfiedHm(t)=(H(t)−m _(av)(t))/m _(av)(t)  [Equation 3]:Sn(t)=(S(t)−n _(av)(t))/n _(av)(t)  [Equation 4]: where m_(av)(t) is anaverage value of a time function of the signal intensity of the thermalradiation light of a plurality of times of welding determined to have noabnormality in the welded portion within the determination period,n_(av)(t) is an average value of a time function of the signal intensityof the plasma light of the plurality of times of welding within thedetermination period, H(t) is a time function of the signal intensity ofthe thermal radiation light before being normalized within thedetermination period, S(t) is a time function of the signal intensity ofthe plasma light before being normalized within the determinationperiod, Hm(t) is a time function of the normalization signal of thethermal radiation light within the determination period, and Sn(t) is atime function of the normalization signal of the plasma light within thedetermination period.
 7. A laser welding quality inspection apparatusfor a welded portion between a joining object and a joined object, whenthe joining object and the joined object are welded by being irradiatedwith a laser beam, the apparatus comprising: a measurement device; and awelding state determination device, wherein the welding statedetermination device includes a signal intensity acquisitor thatacquires, from the measurement device, first data indicating a signalintensity of thermal radiation light radiated from the welded portionduring welding, and second data indicating a signal intensity of plasmalight radiated from the welded portion during the welding, and a signalintensity processor that executes processing of the first data and thesecond data acquired by the signal intensity acquisitor, and wherein thesignal intensity processor determines whether or not the welded portionincludes an abnormality based on a comparison between the signalintensity of the thermal radiation light and the signal intensity of theplasma light which are acquired.
 8. The laser welding quality inspectionapparatus of claim 7, wherein the signal intensity processor calculatesa difference signal indicating a difference between the signal intensityof the thermal radiation light and the signal intensity of the plasmalight based on the first data and the second data, and determines thatthe welded portion includes an abnormality when the calculateddifference signal includes a peak having a signal intensity larger thana preset determination reference value.
 9. The laser welding qualityinspection apparatus of claim 8, wherein the signal intensity acquisitorfurther acquires an irradiation output waveform indicating an intensityof irradiation light of the laser beam from the measurement device,wherein the signal intensity processor sets, as a determination period,a period during which the intensity of the irradiation light of thelaser beam is constantly maintained based on the irradiation outputwaveform, and extracts the signal intensity of the thermal radiationlight within the determination period and the signal intensity of theplasma light within the determination period respectively from thesignal intensity of the thermal radiation light and the signal intensityof the plasma light, and wherein the difference signal indicates adifference between the signal intensity of the thermal radiation lightwithin the determination period and the signal intensity of the plasmalight within the determination period.
 10. The laser welding qualityinspection apparatus of claim 9, wherein the signal intensity processorcalculates a normalization signal of the thermal radiation light and anormalization signal of the plasma light by respectively normalizing thesignal intensity of the thermal radiation light within the determinationperiod and the signal intensity of the plasma light within thedetermination period, and wherein the difference signal indicates adifference between the normalization signal of the thermal radiationlight and the normalization signal of the plasma light.
 11. The laserwelding quality inspection method of claim 1, wherein the signalintensity of the thermal radiation light includes a first thermalradiation light intensity indicating an intensity of the thermalradiation light at a first time point during the welding, and a secondthermal radiation light intensity indicating an intensity of the thermalradiation light at a second time point different from the first timepoint during the welding, wherein the signal intensity of the plasmalight includes a first plasma light intensity indicating an intensity ofthe plasma light at the first time point and a second plasma lightintensity indicating an intensity of the plasma light at the second timepoint, and wherein the comparison between the signal intensity of thethermal radiation light and the signal intensity of the plasma lightincludes calculating a first difference value indicating a differencebetween the first thermal radiation light intensity and the first plasmalight intensity, and a second difference value indicating a differencebetween the second thermal radiation light intensity and the secondplasma light intensity, and generating a difference signal including thefirst difference value and the second difference value.
 12. A laserwelding quality inspection apparatus comprising: a processor; and amemory storing a program, wherein when the program is executed, theprocessor performs acquiring, from a first sensor, first data indicatinga signal intensity of thermal radiation light radiated from a workpiecethat has received a laser beam during laser welding, acquiring, from asecond sensor, second data indicating a signal intensity of plasma lightradiated from the workpiece during the laser welding, and determiningwhether or not a welded portion of the workpiece includes an abnormalitybased on a comparison between the signal intensity of the thermalradiation light and the signal intensity of the plasma tight.